Very simplistically, instead of working out what the loads on a structure
are likely to be and then making that structure twice as strong (elastic
design) designers worked out what load would make them collapse, and then
make them able to withstand twice that load.

Elastic theory does not accurately model the stresses in real buildings
because the equations are very sensitive to small anomalies in a real
structure. Thus a small settling of the foundations or a slight error
in the manufacture of the length of a steel girder can give very different
stresses to those calculated. That is why Baker considered elastic solutions
not to be a proper design tool for structural frames.